Method of making high strength aluminum alloy forgings and product produced thereby

ABSTRACT

An aluminum base alloy containing zinc, magnesium and copper is fabricated into a forging having very high strength. The alloy is cast, homogenized and fabricated into wrought forging stock, preferably by extrusion, under specially prescribed conditions. This stock is soaked, forged, solution heat treated and aged under herein prescribed conditions.

United States Patent [191 Kroger Feb. 12, 1974 METHOD OF MAKING HIGH STRENGTH ALUMINUM ALLOY FORGINGS AND PRODUCT PRODUCED THEREBY lnventor: Paul W. Kroger, San Gabriel, Calif.

Assignee: Aluminum Company of America,

Pittsburgh, Pa.

Filed: Oct. 24, 1972 Appl. No.: 300,153

Related U.S. Application Data Continuation-impart of Ser. No. 164.912, July 21, 1971, abandoned, which is a continuation-in-part of Ser. No. 823,997, May 12, 1969, abandoned.

U.S. Cl 148/2, 72/364, 75/141, 148/115 A, 148/127, 148/32, 148/325 Int. Cl. C22f 1/04 Field of Search... 148/2, 11.5 A, 12.7, 159, 32,

[5 6] References Cited UNITED STATES PATENTS 3,198,676 8/1965 Sprowls et a]. 148/159 Primary Examiner-Richard 0. Dean Attorney, Agent, or Firm-Carl R. Lippert [57] ABSTRACT 40 Claims, No Drawings METHOD OF MAKING HIGH STRENGTH ALUMINUM ALLOY FORGINGS AND PRODUCT PRODUCED THEREBY This is a continuation-in-part of U.S. Ser. No. 164,912, filed July 21, 1971, now abandoned, which, in turn, was a continuation-in-part of US. Ser. No 823,997, filed May 12, 1969, now abandoned.

BACKGROUND OF THE INVENTION The aluminum forging industry has for some time made wide use of an alloy containing 5.1 to 6.1% zinc, 2.1 to 2.9% magnesium, 1.2 to 2% copper, 0.18 to 0.35% chromium with the following limits on impurities: 0.3% Mn, 0.2% Ti, 0.4% Si and 0.5% Fe. This alloy carries the Aluminum Association designation 7075 and is noted for its comparatively high strength and other useful properties among which is very good resistance to stress corrosion cracking when specially aged for such. The special aging treatment referred to is that set out in US Pat. No. 3,198,676 which specifies a two-step aging treatment to improve resistance to stress corrosion cracking while lowering the strength properties somewhat. While forgings of this alloy and other 7000 type alloys are highly useful in the aircraft and space industries because of their strength, their usefulness would be further enhanced by a significant strength improvement which is the purpose of this invention.

DESCRIPTION In practicing the invention, the aluminum alloy is cast, preferably continuously cast, into an ingot which preferably has a maximum cross section dimension of l7 inches. The aluminum alloy according to a preferred embodiment consists essentially of 5.6 to 7% zinc, 2.4 to 2.75% magnesium, .1 .4' to 1.9% copper, 0.18 to 0.35% chromium, with the balance being essentially aluminum together with other elements or impurities with the following maxima applying thereto: 0.1% Mn, 0.1% Ti, 0.12% Si and 0.15% Fe. This alloy is preferred since it consistently develops outstanding properties when produced as forgings under the improved practice. In a broader sense, however, the invention contemplates other alloys since it is believed that many of the benefits of the present invention can be achieved with these other alloys. Accordingly, the invention broadly contemplates a composition consisting essentially of 4.8 to 8.5% zinc, 1.7 to 3.5% magnesium, 0.8 'to 2.5% copper, the balance being aluminum and incidental elements and impurities together with at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium and 0.05 to 0.3% molybdenum. Silicon should be limited to 0.12% and iron to 0.15% as impurities. On a less preferred basis the invention contemplates up to 3% copper although as copper exceeds 2.5% the extent of the improved properties diminishes. During the ingot casting operation, the gas content of the molten metal within the mold is limited to a maximum of 0.15 ml. per 100 grams of melt. Bubbling dry nitrogen through the melt is helpful in this respect as are other known procedures which are applied to the extent required to reduce the melt gas content to the prescribed level. It is desired that the resulting ingot be substantially free from porosity as revealed by a dye-penetrant examination of the ingot cross section and this is the purpose of carefully controlling the molten metal gas content. The dye-penetrant examination referred to is of the type described in Military specification MIL-l-6866B under Inspection Type 11, Method A. In casting the ingot, the chill rate through the solidification temperature zone is maintained at a level of at least 2F per second and preferably at least 10F per second. Maintaining this chill rate through the solidification temperature zone, the zone between the liquidus and solidus temperature, usually requires a most drastic cooling of the metal. The aforementioned chill rates result in the ingot having a desirable dendritic cell size of 0.0030 inch maximum. The faster chill rates are preferred and may reduce the dendritic cell size still further to a maximum of 0.0020 or, better yet, 0.0010 inch. As indicated above, the ingot preferably does not have a cross sectional dimension greater than 17 inches. Still better results are effected where the ingot cross section does not exceed 9 inches in any dimension. For instance, a circular ingot having a diameter of 9 inches is very useful.

The ingot is homogenized by heating to a temperature of at least 860F. It is preferred that it be heated to 860 to 880F for 24 or 48 hours or more to properly homogenize its internal structure. While 24 to 48 hours or more represents an increase over the time normally employed, 8 hours, it is preferred as a precaution to assure the desired results.

The ingot after any necessary scalping or other pro cedures is worked into forging stock, preferably by extrusion, at a temperature of at least 750F, preferably at least 790F, which temperature is maintained throughout the extrusion process. That is, the ingot is not merely heated to a desired minimum temperature prior to working at an uncontrolled and often lower temperature, but rather care is taken to be absolutely sure that the minimum temperature throughout the working cycle is 750 or preferably 790F. This temperature level is somewhat above those normally maintained, about 600F in the case of the extrusion process, but is essential to achieve the desired improvement. Where extrusion is employed it is preferred that the portion of the extrusion first emerging from the extrusion press be removed and discarded. The discarded portion of the extrusion in some cases preferably should amount to at least 20% of its total length so that the remaining portion, constituting a maximum of of the total extrusion length, is employed as the forging stock in the preferred practice. While extrusion is a preferred working operation to produce the forging stock the invention contemplates other operations as well. For instance, a forging or rolling procedure can be employed especially where a flatter or more planar forged product is desired, although some decrease in properties may result over the preferred practice employing extrusion. Compensation for such reduced properties may be achieved for forgings in the stress corrosion resistant temper by adjustments to aging conditions. For example, when using hand forged stock less aging is required to develop the required resistance to stress corrosion cracking than when using extruded forging stock.

Regardless of the particular working operation employed to produce the forging stock it is important that the working be rather extensive or severe such that deformation ratio must be at least 8:1 which, in the case of extruding, rolling and other simplecases, means that the cross sectional area of the metal normal. to the principal direction of working before working is at least 8 times that of the worked stock measured normal to its major axis. In a broader sense a deformation ratio of 8:1 means that the length of the dimension or axis which is lengthened or extended the most during the working operation is at least 8 times that dimension or axis prior to working. It becomes immediately apparent that an extrusion ratio of at least 8:.1 agrees 'with a deformation ratio of at least 8:1 and the latter is considered as encompassing the former. As an example of a forging procedure to prepare the forging stock, a scalped ingot approximately 20by 20 inches in cross section and about 40 inches long is stood upright in a forging press and squeezed to reduce its 40 inch dimension. The resulting biscuit is about 20 inches high and about 30 inches across. This biscuit is then drawn out in a hand forging operation to produce a slab about 160 inches in length, 30 inches in width and a little over 3 inches in thickness. The 160 inch length in comparison with the original starting dimension of 20 inches represents a deformation ratio of 8: 1. It should be noted that the decrease in thickness here was from 40 inches down to a little over 3 inches but this does not represent the deformation ratio which is based on the dimension which has been increased to the greatest extent. As an example of a rolling procedure to prepare forging stock, a scalped ingot approximately 20 by 20 by 40 inches is rolled along the 40 inch dimension to provide a plate 20 by 2 by 400 inches. The deformation ratio is :1 based on the ratio of the final length, 400 inches, versus the starting length of 40 inches. In the case of the rolling operation just discussed and in the case of the extrusion operation mentioned in the preceding paragraph, the deformation ratio does correspond to a reduction ratio but, it will be appreciated, such is not always the case and the deformation ratio aspect is a moreaccurate concept in delineating the practices contemplated .by the invention in a broader or more comprehensive sense where operations such as forging are contemplated to prepare the forging stock. Of course the working operation employed to produce the forging stock must be carried out under conditions which maintain a metal temperature of at least 750F, preferably at least 790F, substantially throughout the entire working operation.

Prior to forging, the wrought forging stock is heated to a temperature of at least 900F, preferably 925 to 970F or even more, for a period of at least 2 hours, preferably at least 4 or 6 or 8 hours. This temperature is well above the solidus temperature of the high strength 7000 type alloys, typically 890F for 7075 alloy, and accordingly care must be taken to avoid incipient melting. A maximum heating rate of 150F per hour from 870F to the hold temperature provides satisfac- -tory results. It is worth mentioning thatno high temperature soak is normally employed with 7075 and other 7000 typev alloy forging stock.

After soaking, the forging stock, which can be segmented in desired lengths, is forged at temperatures of at least 750F, preferably at least 790F. The temperature of the forging stock and the forging tools are all maintained so as to provide actual metal working temperatures at the required level. Normally, 7075 and similar alloys are forged under conditions where some metal temperature may drop well below 600F during the metal working stages of the forging operations. This must be avoided in practicing the invention and care must be taken to positively assure maintenance of the required minimum metal working temperature. The basic characteristics of the forging operation, i.e., pressing between opposing surfaces, are known and need not be elaborated upon here. The invention is especially suited to the production of die forgings for two reasons. First, when the metal is confined by a die cavity there is less tendency for the metal to crack during working at the relatively high temperatures required by the improved process than when making simple shapes designated hand forgings between flat dies which do not confine the metal on all surfaces. Secondly, metal working temperatures are more easily controlled during the single stroke forging operations used for most die forgings than during the multiple stroke operations used in making hand forgings. However, the invention is also applicable to hand forgings in which case the soaking step at 925F or more can be employed either on the starting material when using wrought stock or after forging when forging directly from ingot. When direct forging from ingot, properties will be reduced under the preferred practice where extrusion or the like is employed to produce the forging stock prior to soaking. By hand forgings is meant compressing between opposing surfaces, normallysubstantially flat surfaces, which do not confine metal movement transverse to the direction of forging.

While metal working temperatures below 790F, down to 750F, are permissible in both working the material into forging stock and in the forging operation itself, a major portion, preferably the predominant portion, of the work should be performed at metal temperatures of 790F or more. For instance, in providing forging stock a billet can be introduced to an extrusion press at a temperature of 750F, or perhaps even a little less. If the extrusion is effected rapidly enough, sufficient energy is imparted to the metal to very rapidly raise its temperature significantly over 790F.

The forgings may be solution heat treated, preferably by heating to a temperature of at least 900F and preferably at'least 925F or 940F or more up to 970 or 980F, for a time sufficient to place substantially all the soluble alloy constituents in solid solution. A time of 4 hours, preferably 6 or 8 hours, is usually adequate. This heat treatment temperature is considerably above the 880F level normally employed with 7075 alloy forgings but is deemed necessary in practicing the invention to achieve the best results. Next, the forgings are quenched, preferably in a fluid media maintained at a temperature of less than F which is somewhat below the level of to F normally used for high strength 7000 alloy forgings.

The improved method, as described tov this point, contemplates two separate thermal exposures of sustained duration, at least two hours and preferably at least 4 or 6 or 8 hours, at a temperature of at least 900F, preferably at least 925F, up to 950F or even 970F or more. The exposures referred to are the soaking of the wrought forging stock after the 8:1 deformation working operation and the solution heat treatment. The use of these two separate extended thermal exposures is a preferred practice to help assure the best possible properties. However to some extent one exposure, especially the first, can be traded off against the other. For instance the soaking period of the wrought forging stock before the final forging operation can be reduced, even to below 2 hours, or still further, even eliminated, provided the solution heat treatment is of sufficient temperature and duration. Thus in a broad sense the invention contemplates at least one sustained or extended exposure to a temperature of at least 900F, but preferably at least 925, or better 940F or more. However two or more such exposures, particularly the two described exposures, are preferred. By a sustained exposure is meant one of substantial duration, suitably at least 2 hours and preferably at least 4 or 6 or at least 8 hours.

After solution heat treating and quenching, the forgings are artificially aged to develop the desired strength and other properties. Where strength is the prime consideration, the forgings are aged to a T6 type temper. One preferred practice within the invention achieves the T6 temper by heating to a temperature within the range of 215 to 250F for a minimum period of 50 hours and preferably for a minimum period of 70 hours, for instance, a period of 70 to 75 hours. This is a substantial departure. from the aging treatment normally employed for the T6 type temper, 24 hours at Y 250F, but is desired to achieve good results.

Alternatively, the forgings may be treated according to the procedure of US. Pat. No. 3,198,676 in order to achieve very high resistance to stress corrosion cracking with some sacrifice in strength. Typically for the preferred alloy composition, the forgings after quenching are subjected to a first aging treatment at a temperature within the range of 215 to 250F for a minimum period of 6 hours followed by a second aging treatment manner. Table l compares the tensile properties of the improved high strength forgings (H. S.) with those of ordinary 7075 alloy forgings. In the production of the improved fo rgings an ingot was cast in accordance with the improvement and the ingot was homogenized at a temperature of about 900F. The ingot was scalped and cut to provide extrusion stock which was extruded at an extrusion ratio of about 10:] while maintaining a metal temperature of over 800F. The extrusion was then soaked for about 6 hours at 950F and then forged at just over 800F. The forgings were solution heat treated at approximately 925F, quenched and artificially aged to the T6 type temper. The standard 7075 forgings were produced according to standard forging practices. lncluded are tensile strength (T. 5.), yield strength (Y. S.) and percent elongation in 2 inches for specimens taken both in the longitudinal direction (the direction of maximum metal flow and longest grain dimension) and for specimens taken in a direction transverse to the longitudinal direction. The comparison includes both the higher strength (T6 type) temper and the highly stress corrosion cracking resistant (T73 type) temper. The zinc content for the improved forgings described in Table l was between 5.6 and 6% which is a preferred range from the standpoint of ductility in the T6 type temper. The composition of the improved forgings was otherwise also in accordance with the preferred alloy set forth in the early part of this description. The figures in Table I need little elaboration. The strength improvements for the most part substantially exceed 10% particularly in the longitudinal direction.

Subsequent testing of hundreds of forgings have con- TABLE I Longitudinal Direction Transverse Direction Material and Strength, ksi Elongation Strength, ksi Elongation Temper T.S. Y.S. T.S. Y.S.

7075T6 75 65 7 71 62 3 H.S.T6 86 76 7 77 66 4 7075T73 66 56 7 62 53 3 H.S.-T73 76 66 7 H 62 4 where the temperature is maintained within therange of 340 to 360F for a sufficient period of time to impart the aging effects equivalent to a 7 V2 to 9 Va hour treatment at a temperature of exactly 350F. The nominal treatment of 8 /2 hours at 350F can be roughly translated to a time of about 13 hours at 340F or S-Vz hours at 360F with similarly apportioned hold times for intermediate temperatures. This results in a temper which can be designated the T 73 type temper because of its diminished strength but greatly improved resistance to stress corrosion cracking. The forgings in this temper exhibit an electrical conductivity of 38 to 42% of the International Annealed Copper Standard (IACS) which is a characteristic indication of the T73 type conditions. The above times at 350F are illustrative in that the optimum time can vary from one forged shape to another and should be determined for a particular shape either empirically or from experience with similar shapes. This time can vary from 6 to ll hours for die forgings and from 3 a to 7 hours for hand forgings.

Forgings produced in accordance with the improved method exhibit marked strength improvements over ordinary 7075 forgings. Furthermore, this improvement is realized in a highly repeatable and consistent firmed that the improved process will develop the properties shown in the table for the H. S. forgings. Statistical analysis of test data showed confidence that 99% of test values actually exceed the values in Table l for H. S. forgings.

In another example, an alloy containing about 5.9% Zn, 2.5% Mg, 1.6% Cu, 0.12% Zr, balance aluminum was formed into die forgings according to two different procedures. The first procedure embodying the present invention included casting the ingot in accordance with the improvement to provide extrusion stock, which was homogenized, extruded, soaked and forged in accordance with the improvement. The second included standard practices. Table ll below compares the minimum tensile properties of the forgings produced in accordance with the present improvement vs. those pro duced by standard methods. Minimum tensile properties are shown since such offers one critical aspect of comparison useful to commercial suppliers and purchasers of forgings in that the guaranteed properties are associated with the minimum rather than the normal or average properties. Hence, a comparison of minimum properties is of very substantial importance in weighing the merit of a fabrication procedure. The

table-below compares the longitudinal properties of the two forgings.

TABLE II Material and Temper Strength, ksi

- T.S. Y.S.

HS. 75 68.2 STD. 69 62.6

such-that all the forgings produced according to the improvement reliably exhibit higher strength levels and substantially none exhibit low strength levels. This represents a very substantial improvement over previous practices where, not only were the properties of a generally lower level, but equally significantly the properties were spread through a wide range of levels. This necessitated either guaranteeing properties only to the lowest level or guaranteeing properties to a higher level but then suffering very high rejection rates since many of the forgings then could not meet the higher guaranteed levels. The improvement facilitates guaranteeing still higher levels and achieving at these higher levels little or no rejections. This enables the production of very high strength forgings at an attractive cost.

The invention has been described in terms of certain preferred embodiments but the scope of this description and the claims appended hereto is intended to encompass all embodiments within the spirit of the invention.

Having thus described my invention and certainembodiments thereof, I claim:

I. In the method of producing a high strength aluminum forging, the steps comprising:

1. providing a body having a composition consisting essentially of 4.8 to 8.5% zinc, 1.7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium, 0.05 to 0.3% molybdenum, the balance being substantially aluminum, said alloy being cast while maintaining in the molten metal phase within the casting mold a maximum gas content of 0.15 ml/l g of melt to achieve substantial freedom from porosity, the metal solidification being controlled such that the chill rate throughout the solidification temperature zone is a minimum of 2F per second,

2. working said body. at a deformation ratio of at least 8:] while maintaining in the metal a temperature of at least 750F, said working including at least some forging,

3. subjecting said body. to at least one sustained exposure to a temperature of at least 900F.

2. In the method of producing a high strength aluminum forging, the steps comprising:

1. providing a body having a composition consisting essentially of 4.8 to 8.5% zinc, 1.7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium, 0.05 to 0.3% molybdenum, the balance being substantially aluminum, said alloy being cast while maintaining in the molten metal phase within the casting mold a maximum gas content of 0.15 ml/l00 g of melt to achieve substantial freedom from porosity, the metal solidification being controlled such that the chill rate throughout the 'solidification temperature zone is a minimum of 2F per second,

2. forging said body at a deformation ratio of at least 8:1 while maintaining in the metal a temperature of at least 750F,

3. subjecting said'body to at least one sustained exposure to a temperature of at least 900F.

3. In the method of producing a high strength aluminum forging, the steps comprising:

1. providing an ingot having a composition consisting essentially of 4.8 to 8.5% zinc, L7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium, 0.05 to 0.3% molybdenum, the balance being substantially aluminum, said ingot being cast while maintaining in the molten metal phase within the casting mold a maximum gas content of 0.15 ml/l00 g of melt to achieve substantial freedom from porosity, the metal solidification being controlled such that the chill rate throughout the solidification temperature zone is a minimum of 2F per second,

2. homogenizing said ingot by heating to a temperature of at least 860F,

3. working said ingot at a deformation ratio of at least 8:1 while maintaining in the metal a temperature of at least 750F to provide forging stock,

to 970F for a period of at least 2 hours,

5. forging the forging stock to provide a forged product while maintaining throughout the forging operation a metal temperature of at least 750F.

4. In the method according to claim 3 the additional steps of solution heat treating the said forged product by heating it to a temperature of at least 900F for a time sufficient to place substantially all the soluble -alloy constituents in solid solution and quenching the forged product.

5. In the method according to claim 4 the additional step of artificially aging said forged product.

6. In the method according to claim 1 wherein major portions of ,said working in said step (2) are performed at metal temperatures of at least 790F.

7. In the method according to claim 2 wherein in said step (2) a major portion of said forging is performed at a metal temperature of at least 790F.

8. In the method according to claim 3 wherein major portions of said working in said step (3) and said forging in said step (5) are performed at metal temperatures of at least 790F.

. soaking said forging stock at a temperature of 900 9. 1n the method according to claim 1 wherein said exposure in said step (3) is at a temperature of at least 925F.

10. In the method according to claim 2 wherein said exposure in said step (3) is at a temperature of at least 925F.

111. 1n the method according to claim 9 wherein said body contains 5.6 to 7% zinc, 2.4 to 2.75% magnesium, 1.4 to 1.9% copper and 0.18 to 0.35% chromium.

12. In the method according to claim wherein said body contains 5.6 to 7% zinc, 2.4 to 2.75% magnesium,

1.4 to 1.9% copper and 0.18 to 0.35% chromium.

13. The method of producing a high strength aluminum alloy forging comprising the steps:

1. casting-an ingot having a composition consisting essentially of 5.6 to 7% zinc, 2.4 to 2.75% magnesium, 1.4 to 1.9% copper, 0.18 to 0.35% chromium, the balance being aluminum with the following maxima on impurities: 0.1% manganese, 0.1% titanium, 0.12% silicon and 0.15% iron, said casting being accomplished while maintaining in the molten metal phase within the mold a maximum gas content of 0.15 ml/100 g of melt, the metal solidification being controlled such that the chill rate through the solidification temperature zone is a minimum of 2F per second, to produce an ingot exhibiting substantial freedom from any porosity as revealed by a dye-penetrant examination of its cross section,

2. homogenizing said ingot by heating to a temperature of at least 860F,

3. working said ingot at a deformation ratio of at least 8 to 1 while maintaining in the metal a temperature of at least 790F to provide forging stock,

' 4.soaking said forging stock at a temperature of 925 to 970F for a minimum time period of 2 hours,

5. forging the forging stock to provide a forging while maintaining throughout the forging operation a metal temperature of at least 790F,

6. solution heat treating said forging by heating it to a temperature within the range of 925 to 980F for a time sufficient to place substantially all the soluble alloy constituents in solid solution,

7. quenching said forging,

8. artificially aging said quenched forging, said forging being characterized by very high strength.

14. The method according to claim 3 wherein said forging is artificially aged by heating to a temperature of 215 to 250F and holding at said temperature for a minimum time of 6 hours and thereafter heating to a temperature within they range of 340 to 360F and holding at said temperature for a duration which imparts the effects ofa 7 A to 9 k hour exposure at a temperature of exactly 350F so as to impart to said forging an electrical conductivity of-38 to 42% lACS, said forging being characterized by high resistance to stress corrosion cracking and by a minimum strength level of 76,000 psi tensile and 66,000 psi yield strength and a minimum elongation of 7% in the longitudinal direction.

15. The method according to claim 3 wherein said forging is artificially aged at a temperature within the range of 215 to 250F for a minimum period of 50 hours, said forging being characterized by a strength level of 86,000 psi tensile and 76,000 psi yield strength and a minimum elongation of 7% in the longitudinal direction.

16. The method according to claim 3 wherein extrusion is employed in producing the forging stock in said step (3).

17. The method according to claim 3 wherein extrusion is employed to produce the forging stock in said step (3) and the first portion of the extruded forging stock is discarded, the discarded portion amounting to at least 20% of the total extruded length.

18. The method according to claim 3 wherein said forging stock, prior to forging, is soaked at a temperature of 925 to 970F for a minimum time of 8 hours.

19. The method according to claim 3 wherein said ingot contains 5.6 to 6% zinc.

20. The method according to claim 1 wherein said body is homogenized at a temperature of at least 860F prior to said working of said step (2).

21. The method according to claim 2 wherein said body is homogenized at a temperature of at least 860F prior to said forging of said step (2).

22. The method according to claim 1 wherein said forging is artificially aged by heating to a temperature of 215 to 250F and holding at said temperature for a minimum time of 6 hours and thereafter heating to a temperature within the range of 340 to 360F and holding at said temperature for a duration which imparts the effects ofa 7 5a to 9 16 hour exposure at a temperature of exactly 350F so as to impart to said forging an electrical conductivity of 38 to 42% lACS, said forging being characterized by high resistance to stress corrosion cracking and by a minimum strength level of 76,000 psi tensile and 66,000 psi yield strength and a minimum elongation of 7% in the longitudinal direction.

23. The method according to claim 1 wherein said forging is artificially aged at a temperature within the range of 215 to 250F for a minimum period of 50 hours, said forging being characterized by a strength level of 86,000 psi tensile and 76,000 psi yield strength and a minimum elongation of7% in the longitudinal direction.

24. A method according to claiml wherein extrusion to a deformation ratio of at least 8:1 is employed in said step (2) to produce stock which is forged.

25. The method according to claim 1 wherein, in said step (2), said body is first worked to a deformation ratio of at least 8:1, and then soaked at a temperature of 925 to 970F for a minimum time of 2 hours and then forged.

26. The method according to claim 1 wherein said ingot contains 5.6 to 6% zinc.

27. In a method of producing an improved high strength forging of an aluminum alloy of the zinc-magnesium-copper type wherein a body of the alloy is forged at a deformation ratio of at least 8:1 while maintaining a metal temperature of at least 750F and is subjected to at least one sustained exposure to a tempera ture of at least 900F, the step of providing said body in an alloy having a composition consisting essentially of 4.8 to 8.5% zinc, 1.7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium, 0.05 to 0.3% molybdenum, the balance being substantially aluminum, said alloy being cast while maintaining in the molten metal phase within the casting mold a maximum gas content of 0. 1 5 ml/100 g of melt to achieve substantial freedom from porosity, the metal solidification being controlled such that the chill rate'throughout the solidification temperature zone is a minimum of 2F per second.

28. In a method of producing an improved high strength forging of an aluminum alloy of the zinc-magnesium-copper type wherein a body of the alloy is worked at a deformation ratio of at least 8: 1 said working including at least some forging, while maintaining a metal temperature of at least 750F, and is subjected to at least one sustained exposure to a temperature of at least 900F, the step of providing said body in an alloy having a composition consisting essentially of 4.8 to 8.5% zinc, 1.7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium, 0.05 to 0.3% molybdenum, the balance being substantially aluminum, said alloy being cast while maintaining in the molten metal phase within the casting mold a maximum gas content of 0.15 ml/l g of meltto achieve substantial freedom from porosity, the metal solidification being controlled such that the chill rate throughout the solidification temperature zone is a minimum of 2F per second.

29. In the method of producing an improved high strength forging of an aluminum alloy of the zinc-magnesium-copper type wherein a body of said alloy is provided having a composition consisting essentially of 4.8 to'8.5% zinc, 1.7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium, 0.05 to 0.3% molybdenum, the balance being substantially aluminum, said alloy being cast while maintaining in the molten metal phase within the casting mold a maximum gas content of 0.15 ml/100 g of melt to achieve substantial freedom from porosity, the'metal solidification being controlled such that the chill rate throughout the solidification temperature zone is a minimum of 2F per second, and the body is worked at a deformation ratio of at least 8:1 while maintaining a metal temperature of at least 750F, the step of subjecting said body to at least one sustained exposure to a temperature of at least 900F.

30. In a method of producing a high strength aluminum alloy forging, the steps comprising:

1. working at a metal temperature of at least 750F a substantially porosity-free body of an aluminum alloy consisting essentially of 4.8 to 8.5% zinc, 1.7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05

to 0.3% vanadium, 0.05 to 0.3% molybdenum, the

balance being substantially aluminum, said working including at least some forging,

2. subjecting the body to at least one sustained exposure to a temperature of at least 900F.

31. In the method according to claim 30 wherein said sustained temperature exposure in said step (2) follows said working in said step (1).

32. In the method according to claim 30 wherein said working imparts a deformation ratio of at least 8:1.

33. The method according to claim 30 wherein said thermal exposure is at a temperature of at least 925F.

34. The method according to claim 1 wherein said body -contains not more than 0.12% silicon and not more than 0.15% iron.

35. The method according to claim 2 wherein said body contains not more than 0.12% silicon and not more than 0.15% iron.

36. The method according to claim 3 wherein said body contains not more than 0.12% silicon and not more than 0.15% iron.

37. The method according to claim 27 wherein said body contains not more than 0.12% silicon and not more than 0.15% iron.

38. The method according to claim 28 wherein said body contains not more than 0.12% silicon and not more than 0.15% iron.

39. An improved forging produced according to the method of claim 30.

40. An improved forging produced by the method of claim 1. 

2. homogenizing said ingot by heating to a temperature of at least 860*F,
 2. In the method of producing a high strength aluminum forging, the steps comprising:
 2. forging said body at a deformation ratio of at least 8:1 while maintaining in the metal a temperature of at least 750*F,
 2. subjecting the body to at least one sustained exposure to a temperature of at least 900*F.
 2. homogenizing said ingot by heating to a temperature of at least 860*F,
 2. working said body at a deformation ratio of at least 8:1 while maintaining in the metal a temperature of at least 750*F, said working including at least some forging,
 3. working said ingot at a deformation ratio of at least 8 to 1 while maintaining in the metal a temperature of at least 790*F to provide forging stock,
 3. subjecting said body to at least one sustained exposure to a temperature of at least 900*F.
 3. In the method of producing a high strength aluminum forging, the steps comprising:
 3. subjecting said body to at least one sustained exposure to a temperature of at least 900*F.
 3. working said ingot at a deformation ratio of at least 8:1 while maintaining in the metal a temperature of at least 750*F to provide forging stock,
 4. soaking said forging stock at a temperature of 900* to 970*F for a period of at least 2 hours,
 4. In the method according to claim 3 the additional steps of solution heat treating the said forged product by heating it to a temperature of at least 900*F for a time sufficient to place substantially all the soluble alloy constituents in solid solution and quenching the forged product.
 4. soaking said forging stock at a temperature of 925* to 970*F for a minimum time period of 2 hours,
 5. forging the forging stock to provide a forging while maintaining throughout the forging operation a metal temperature of at least 790*F,
 5. In the method according to claim 4 the additional step of artificially aging said forged product.
 5. forging the forging stock to provide a forged product while maintaining throughout the forging operation a metal temperature of at least 750*F.
 6. In the method according to claim 1 wherein major portions of said working in said step (2) are performed at metal temperatures of at least 790*F.
 6. solution heat treating said forging by heating it to a temperature within the range of 925* to 980*F for a time sufficient to place substantially all the soluble alloy constituents in solid solutiOn,
 7. quenching said forging,
 7. In the method according to claim 2 wherein in said step (2) a major portion of said forging is performed at a metal temperature of at least 790*F.
 8. In the method according to claim 3 wherein major portions of said working in said step (3) and said forging in said step (5) are performed at metal temperatures of at least 790*F.
 8. artificially aging said quenched forging, said forging being characterized by very high strength.
 9. In the method according to claim 1 wherein said exposure in said step (3) is at a temperature of at least 925*F.
 10. In the method according to claim 2 wherein said exposure in said step (3) is at a temperature of at least 925*F.
 11. In the method according to claim 9 wherein said body contains 5.6 to 7% zinc, 2.4 to 2.75% magnesium, 1.4 to 1.9% copper and 0.18 to 0.35% chromium.
 12. In the method according to claim 10 wherein said body contains 5.6 to 7% zinc, 2.4 to 2.75% magnesium, 1.4 to 1.9% copper and 0.18 to 0.35% chromium.
 13. The method of producing a high strength aluminum alloy forging comprising the steps:
 14. The method according to claim 3 wherein said forging is artificially aged by heating to a temperature of 215* to 250*F and holding at said temperature for a minimum time of 6 hours and thereafter heating to a temperature within the range of 340* to 360*F and holding at said temperature for a duration which imparts the effects of a 7 1/2 to 9 1/2 hour exposure at a temperature of exactly 350*F so as to impart to said forging an electrical conductivity of 38 to 42% IACS, said forging being characterized by high resistance to stress corrosion cracking and by a minimum strength level of 76,000 psi tensile and 66,000 psi yield strength and a minimum elongation of 7% in the longitudinal direction.
 15. The method according to claim 3 wherein said forging is artificially aged at a temperature within the range of 215* to 250*F for a minimum period of 50 hours, said forging being characterized by a strength level of 86,000 psi tensile and 76, 000 psi yield strength and a minimum elongation of 7% in the longitudinal direction.
 16. The method according to claim 3 wherein extrusion is employed in producing the forging stock in said step (3).
 17. The method according to claim 3 wherein extrusion is employed to produce the forging stock in said step (3) and the first portion of the extruded forging stock is discarded, the discarded portion amounting to at least 20% of the total extruded length.
 18. The method according to claim 3 wherein said forging stock, prior to forging, is soaked at a temperature of 925* to 970*F for a minimum time of 8 hours.
 19. The method according to claim 3 wherein said ingot contains 5.6 to 6% zinc.
 20. The method according to claim 1 wherein said body is homogenized at a temperature of at least 860*F prior to said working of said step (2).
 21. The method according to claim 2 wherein said body is homogenized at a temperature of at least 860*F prior to said forging of said step (2).
 22. The method according to claim 1 wherein said forging is artificially aged by heating to a temperature of 215* to 250*F and holding at said temperature for a minimum time of 6 hours and thereafter heating to a temperature within the range of 340* to 360*F and holding at said temperature for a duration which imparts the effects of a 7 1/2 to 9 1/2 hour exposure at a temperature of exactly 350*F so as to impart to said forging an electrical conductivity of 38 to 42% IACS, said forging being characterized by high resistance to stress corrosion cracking and by a minimum strength level of 76,000 psi tensile and 66,000 psi yield strength and a minimum elongation of 7% in the longitudinal direction.
 23. The method according to claim 1 wherein said forging is artificially aged at a temperature within the range of 215* to 250*F for a minimum period of 50 hours, said forging being characterized by a strength level of 86,000 psi tensile and 76, 000 psi yield strength and a minimum elongation of 7% in the longitudinal direction.
 24. A method according to claim 1 wherein extrusion to a deformation ratio of at least 8:1 is employed in said step (2) to produce stock which is forged.
 25. The method according to claim 1 wherein, in said step (2), said body is first worked to a deformation ratio of at least 8:1, and then soaked at a temperature of 925* to 970*F for a minimum time of 2 hours and then forged.
 26. The method according to claim 1 wherein said ingot Contains 5.6 to 6% zinc.
 27. In a method of producing an improved high strength forging of an aluminum alloy of the zinc-magnesium-copper type wherein a body of the alloy is forged at a deformation ratio of at least 8: 1 while maintaining a metal temperature of at least 750*F and is subjected to at least one sustained exposure to a temperature of at least 900*F, the step of providing said body in an alloy having a composition consisting essentially of 4.8 to 8.5% zinc, 1.7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium, 0.05 to 0.3% molybdenum, the balance being substantially aluminum, said alloy being cast while maintaining in the molten metal phase within the casting mold a maximum gas content of 0.15 ml/100 g of melt to achieve substantial freedom from porosity, the metal solidification being controlled such that the chill rate throughout the solidification temperature zone is a minimum of 2*F per second.
 28. In a method of producing an improved high strength forging of an aluminum alloy of the zinc-magnesium-copper type wherein a body of the alloy is worked at a deformation ratio of at least 8: 1, said working including at least some forging, while maintaining a metal temperature of at least 750*F, and is subjected to at least one sustained exposure to a temperature of at least 900*F, the step of providing said body in an alloy having a composition consisting essentially of 4.8 to 8.5% zinc, 1.7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium, 0.05 to 0.3% molybdenum, the balance being substantially aluminum, said alloy being cast while maintaining in the molten metal phase within the casting mold a maximum gas content of 0.15 ml/100 g of melt to achieve substantial freedom from porosity, the metal solidification being controlled such that the chill rate throughout the solidification temperature zone is a minimum of 2*F per second.
 29. In the method of producing an improved high strength forging of an aluminum alloy of the zinc-magnesium-copper type wherein a body of said alloy is provided having a composition consisting essentially of 4.8 to 8.5% zinc, 1.7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium, 0.05 to 0.3% molybdenum, the balance being substantially aluminum, said alloy being cast while maintaining in the molten metal phase within the casting mold a maximum gas content of 0.15 ml/100 g of melt to achieve substantial freedom from porosity, the metal solidification being controlled such that the chill rate throughout the solidification temperature zone is a minimum of 2*F per second, and the body is worked at a deformation ratio of at least 8:1 while maintaining a metal temperature of at least 750*F, the step of subjecting said body to at least one sustained exposure to a temperature of at least 900*F.
 30. In a method of producing a high strength aluminum alloy forging, the steps comprising:
 31. In the method according to claim 30 wherein said sustained temperature exposure in said step (2) follows said working in said step (1).
 32. In the method according to claim 30 wherein said working imparts a deformation ratio of at least 8:1.
 33. The method according to claim 30 wherein said thermal exposure is at a temperature of at least 925*F.
 34. The method according to claim 1 wherein said body contains not more than 0.12% silicon and not more than 0.15% iron.
 35. The method according to claim 2 wherein said body contains not more than 0.12% silicon and not more than 0.15% iron.
 36. The method according to claim 3 wherein said body contains not more than 0.12% silicon and not more than 0.15% iron.
 37. The method according to claim 27 wherein said body contains not more than 0.12% silicon and not more than 0.15% iron.
 38. The method according to claim 28 wherein said body contains not more than 0.12% silicon and not more than 0.15% iron.
 39. An improved forging produced according to the method of claim
 30. 40. An improved forging produced by the method of claim
 1. 